PC Components, Features, and System Design
PCs can be broken down into many categories. I like to break them down in twowaysby the type of software they can run and by the motherboard host bus,or processor bus design and width. Because this book concentrates mainly onhardware, let's look at that first.
When a processor reads data, the data moves into the processor via theprocessor's external data bus connection. The processor's data bus isdirectly connected to the processor host bus on the motherboard. The processordata bus or host bus is also sometimes referred to as the local busbecause it is local to the processor that is connected directly to it. Any otherdevices connected to the host bus essentially appear as if they are directlyconnected to the processor as well. If the processor has a 32-bit data bus, themotherboard must be wired to have a 32-bit processor host bus. This means thesystem can move 32 bits of data into or out of the processor in a singlecycle.
See "Data I/O Bus," p. 46.
Different processors have different data bus widths, and the motherboardsdesigned to accept them require a processor host bus with a matching width.Table 2.2 lists all the Intel and major Intel-compatible processors, their databus widths, and their internal register sizes.
Table 2.2 Intel and Intel-Compatible Processors and Their DataBus/Register Widths
Data Bus Width
AMD Duron/Athlon/Athlon XP
AMD Athlon 64
A common misconception arises in discussions of processorwidths. Although the Pentium and newer processors all have 64-bit data buswidths, their internal registers are only 32 bits wide, and they process 32-bitcommands and instructions. The Intel Itanium and AMD Athlon 64 are the firstIntel-compatible processors to have 64-bit internal registers. Thus, from asoftware point of view, all chips from the 386 to the Athlon/Duron andCeleron/Pentium 4 have 32-bit registers and execute 32-bit instructions. Fromthe electronic or physical perspective, these 32-bit, software-capableprocessors have been available in physical forms with 16-bit (386SX), 32-bit(386DX and 486), and 64-bit (Pentium and beyond) data bus widths. The data buswidth is the major factor in motherboard and memory system design because itdictates how many bits move in and out of the chip in one cycle.
See "Internal Registers (Internal Data Bus)," p. 48.
The Itanium processor has a new Intel architecture 64-bit (IA-64) instructionset, but it can also process the same 32-bit instructions as processors rangingfrom the 386 through the Pentium 4. The Athlon 64 has a new x86-compatible64-bit architecture but is designed to use 32-bit instructions written fornormal Intel or compatible x86 processors as efficiently as a normal Athlon XPor comparable processor would.
See "Processor Specifications," p. 41.
Referring to Table 2.2, you can see that all Pentium and newer systems have a64-bit processor bus. Pentium processors, whether they are the original Pentium,Pentium MMX, Pentium Pro, or even the Pentium II/III or 4, all have 64-bit databuses, as do comparable processors from AMD (K6 family, Athlon, Duron, AthlonXP, and Athlon 64).
As you can see from Table 2.2, systems can be broken down into the followinghardware categories:
What is interesting is that besides the bus width, the 16- through 64-bitsystems are remarkably similar in basic design and architecture. The older 8-bitsystems are very different, however. This gives us two basic system types, orclasses, of hardware:
In this verbiage, PC stands for personal computer; XT stands for an extendedPC; and AT stands for an advanced-technology PC. The terms PC, XT,and AT, as they are used here, are taken from the original IBM systems ofthose names. The XT was a PC system that included a hard disk for storage inaddition to the floppy drives found in the basic PC system. These systems had an8-bit 8088 processor and an 8-bit Industry Standard Architecture (ISA) bus forsystem expansion. The bus is the name given to expansion slots in whichadditional plug-in circuit boards can be installed. The 8-bit designation comesfrom the fact that the ISA bus found in the PC/XT class systems can send andreceive only 8 bits of data in a single cycle. The data in an 8-bit bus is sentalong eight wires simultaneously, in parallel.
See "The ISA Bus," p. 350.
16-bit and greater systems are said to be AT-class, which indicates that theyfollow certain standards and that they follow the basic design first set forthin the original IBM AT system. AT is the designation IBM applied to systems thatfirst included more advanced 16-bit (and later, 32- and 64-bit) processors andexpansion slots. AT-class systems must have a processor that is compatible withIntel 286 or higher processors (including the 386, 486, Pentium, Pentium Pro,Pentium II, Pentium III, Pentium 4, and Pentium M processors), and they musthave a 16-bit or greater system bus. The system bus architecture is central tothe AT system design, along with the basic memory architecture, interruptrequest (IRQ), direct memory access (DMA), and I/O port address design. AllAT-class systems are similar in the way these resources are allocated and howthey function.
The first AT-class systems had a 16-bit version of the ISA bus, which is anextension of the original 8-bit ISA bus found in the PC/XT-class systems.Eventually, several expansion slot or bus designs were developed for AT-classsystems, including the following:
16-bit ISA/AT bus
16-bit PC Card (PCMCIA) bus
16/32-bit Extended ISA (EISA) bus
16/32-bit PS/2 Micro Channel Architecture (MCA) bus
32-bit VESA Local (VL) bus
32/64-bit Peripheral Component Interconnect (PCI) bus
32-bit CardBus (PCMCIA) bus
PCI Express bus
32-bit Accelerated Graphics Port (AGP) bus
A system with any of these types of expansion slots is by definition anAT-class system, regardless of the actual Intel or Intel-compatible processorthat is used. AT-type systems with 386 or higher processors have specialcapabilities not found in the first generation of 286-based ATs. These distinctcapabilities are in the areas of memory addressing, memory management, andpossible 32- or 64-bit wide access to data. Most systems with 386DX or higherchips also have 32-bit bus architectures to take full advantage of the 32-bitdata transfer capabilities of the processor.
Until recently, PC systems continued to incorporate a 16-bit ISA slot forbackward-compatibility and lower-function adapters. However, in virtually allmotherboards today, ISA slots have been completely replaced by PCI slots alongwith an AGP slot (a specialized expansion slot design) available in most systems(except for a few entry-level models with integrated video) for high-performancegraphics. In addition, most portable systems use PC Card (PCMCIA) and CardBusslots in the portable unit and PCI slots in optional docking stations.
Chapter 4, "Motherboards and Buses," contains in-depth informationon these and other PC system buses, including technical information such aspinouts, performance specifications, and bus operation and theory.
Table 2.3 summarizes the primary differences between the older 8-bit (PC/XT)systems and modern AT systems. This information distinguishes between thesesystems and includes all IBM and compatible models.
Table 2.3 Differences Between PC/XT and AT Systems
(8-Bit) PC/XT Type
(16/32/64-Bit) AT Type
All x86 or x88
286 or higher
16- or 32-bit
Bus slot width
ISA, EISA, MCA, PC Card, CardBus, ExpressCard, VL-Bus, PCI, PCI Express, andAGP
8 (6 usable)
16 (11 usable)
4 (3 usable)
8 (7 usable)
16MB/4GB or more
Floppy controller speed
Standard boot drive
360KB or 720KB
16450/16550A or greater
The easiest way to identify a PC/XT (8-bit) system is by the8-bit ISA expansion slots. No matter which processor or other features thesystem has, if all the slots are 8-bit ISA, the system is a PC/XT. AT (16-bitplus) systems can be similarly identifiedthey have 16-bit or greater slotsof any type. These can be ISA, EISA, MCA, PC Card (formerly PCMCIA), CardBus,VL-Bus, or PCI. Any system using the new high-speed serial buses such as PCIExpress or ExpressCard also qualifies as an AT-class system. Using thisinformation, you can properly categorize virtually any system as a PC/XT type oran AT type. No PC/XT type (8-bit) systems have been manufactured for many years.Unless you are in a computer museum, virtually every system you encounter todayis based on the AT-type design.